1. Field of the Invention
The present invention relates to a tool holder with a variable tool rotation radius, a machine tool provided with the tool holder, and a machining method using the machine tool.
2. Description of the Related Art
A high-precision mold is used to mass-produce lenses for optical components. Since conventional lenses have a rotationally symmetrical shape, ultra-precision lathe turning using a single-crystal diamond tool is applied to the manufacture of molds. In conventional lathe turning, a workpiece mounted on a spindle is rotated at high speed, and a tool is pressed against the workpiece to cut an arbitrary rotationally symmetrical shape. Thus, there is only one center of rotation on the workpiece.
Recently, however, there has been an increasing demand for lens-array molds (see FIGS. 19A to 20B) each configured so that tens to thousands of lenses with a diameter of several millimeters are arranged side by side. In order to machine a lens-array mold by lathe turning, the rotation center of each lens shape needs to be aligned with the spindle of a lathe.
Since it is difficult to manually adjust a machining position for a lens with precision, a two-axis linear-motion table, for example, is mounted on the spindle so as to be perpendicular to a rotation center axis. The center of rotation on a workpiece can be arbitrarily changed if the workpiece is mounted on the table.
However, a drive cable cannot be connected to the rapidly rotating spindle, and in addition, it is technically difficult to provide the table with a sufficient retention to resist centrifugal force produced during spindle rotation. Alternatively, a large number of lens molds may be separately fabricated using a lathe and combined together into a lens-array mold. In many applications of lens arrays, however, distances between individual lenses are exactly designed, so that it is difficult to assemble thousands of molds with exact distances between the lenses.
Accordingly, there is an eager demand for a high-speed, high-precision machining method for lens arrays other than lathe turning. Milling is generally known as a machining method for lens-array shapes. In this method, a small-diameter rotary tool is mounted on a spindle, and a lens shape is machined by simultaneously driving three orthogonal axes of a machine tool to depict a spiral trajectory.
In machining a complicated free curved surface by conventional milling, as shown in FIG. 25, a small-diameter end mill is used to deal with a small-radius concave portion, if any, in a shape to be machined. Since the small-diameter tool can cut little while it is making one revolution, however, its machining efficiency for a gently sloping surface is very poor. In some ultra-precision machining, in particular, the tool position is not allowed to be shifted even by only 1 μm by tool replacement. In finish machining, therefore, tool replacement is prohibited, so that the entire surface to be machined is bound to be machined by a single tool, in many cases.
FIGS. 26A and 26B are views illustrating lens-array machining by conventional milling. FIG. 26A shows a method in which a cutting tool is rotated by a spindle as a spiral trajectory for machining is depicted by linear axes of a machine tool. FIG. 26B shows a method in which the tool is moved for machining in a scanning manner by the linear axes. In either method, acceleration and deceleration of each axis become sharp if the machining speed is increased, and the machining speed is determined depending on the acceleration performance of the linear axes. Since the tool depicts the trajectories shown in FIGS. 26A and 26B while rotating at high speed, moreover, actual cutting distances of the tool are much longer than the trajectories, and the tool is significantly worn.
As described above, a drawback of this machining method lies in that if the machining speed is increased, a fast spiral motion is performed within a lens diameter as small as several millimeters, so that the linear axes frequently change their courses. In machining near the center of a lens, in particular, high-speed switching between acceleration and deceleration is required, so that the machining speed is greatly affected by the acceleration performance of the linear axes. In milling, moreover, the tool is rotated at high speed, so that it is worn more significantly than in lathe turning. Thus, it is difficult to machine thousands of lens shapes without tool replacement.
As a method for suppressing tool wear, there is proposed a method in which the tool angle is changed by a turntable as a spiral motion is made by three orthogonal axes. FIGS. 27A and 27B illustrate another prior art machining method in which the angle of a tool 2 is changed as a spiral trajectory is depicted for lens-array machining. In this machining method, the motion of the tool 2 is similar to that in the case of lathe turning, so that tool wear can be effectively suppressed. Like the methods shown in FIGS. 26A and 26B, however, this method has a drawback that high-speed machining requires sharp acceleration and deceleration of linear axes that make a spiral motion.
Japanese Patent Application Laid-Open Nos. 2003-121612 and 2000-52217 disclose machining methods in which both tool wear and high-speed drive of linear axes are suppressed.
FIGS. 28A and 28N are views illustrating another prior art machining method (Japanese Patent Application Laid-Open No. 2003-121612) in which a workpiece is machined by a tool shaped after the cross-sectional shape of a lens. According to this method, a tool 2 for machining the workpiece is finely moved up and down by means of a piezoelectric element. This method has drawbacks that it is not applicable to convex shapes and that it, in principle, cannot be used to machine rotationally symmetrical shapes, if concave.
FIG. 29 is a view illustrating a prior art technique (Japanese Patent Application No. 2000-52217) in which a workpiece is machined by a tool shaped after the cross-sectional shape of a lens. This technique is applicable to rotationally symmetrical shapes and convex shapes. This technique, however, is not suitable for high-precision lens shape machining, since it is very difficult to mold a tool 2 rotatable around a rotation center axis 4 to a contour precision of micron order or less.
Any of the prior art techniques disclosed in the above-described patent documents is not applicable to the manufacture of high-precision lens molds, since it requires a special tool shaped after the cross-sectional shape of a lens, its shaping precision depends on the tool precision, and it cannot correct shape errors.